1. Field of the Invention
This invention relates to a UV, or electron beam, or visible light curable coating composition suitable for use in providing a protective barrier coating on electrical and electronic components and as an ink for printing on various substrates.
2. Description of the Prior Art
Electrical and electronic components, e.g., thick and thin film hybrid circuits, tantalum capacitors, axial leaded film-foil capacitors, ceramic disk and tubular capacitors, electrolytic capacitors, radial film foil capacitors, resistors, stacked film capacitors, and related electrical and electronic components must be, in general, provided with a protective coating. Such a coating must provide ease in handling and be capable of imparting to the electrical and electronic component certain desired physical, thermal, electrical insulation and moisture resistance properties. The variety of different shapes of electrical and electronic components that are provided with a protective coating present certain unique problems. The composition used must not only provide the desired properties mentioned as a protective coating, it must also be one that can be applied to the component involved using the most suitable coating procedure. Moreover, as with any coating composition, it is desired that the protective coating be obtained in the mos economical manner without compromise to the various properties desired.
In general, the protective barrier coating on an electrical or electronic component is provided by first coating the electrical and electronic component, or a portion thereof, with a resinous composition, and then heating the coated component at a suitable temperature and for a suitable length of time; whereby the coating composition is cured and adheres to the underlying electrical or electronic component. The post curing step of the coating operation may be shortened in some cases by preheating the component before application of the resinous coating composition. The coating composition used may be, in some cases, either a solid, powdery composition, or, in other cases, one that is liquid, depending somewhat upon the substrate being coated. The use of such a coating composition and procedure is, however, attendant with certain disadvantages. In particular, where the resinous composition is solvent based, this creates environmental considerations. Moreover, such compositions sometimes result in the creation of bubbles and pin-holes in the coating layer created by entrapped air during the curing cycle. Furthermore, conventional thermal curing of resinous coating compositions is not only time consuming but also costly in terms of energy consumption, space, equipment, coating material usage and personnel.
Printing and screening inks are applied to a wide variety of substrates, e.g., metals, metal alloys, paper, thermoplastic and thermosetting resin layers, etc. Nevertheless, particular problems are presented when the substrate is heat sensitive. The ink composition applied must be capable of being cured under conditions not damaging to the substrate.
The ultraviolet curing of coating compositions has been known now for sometime. Exemplary of the prior art are U.S. Pat. Nos. 3,896,014; 4,349,605; and 4,390,401. U.S. Pat. No. 3,896,014 discloses liquid nail lacquer compositions which comprise as the essential components a polyene, a polythiol, a photocuring rate accelerator and, as disclosed by the patentee, a surfactant from a particular class. Among the preferred surfactants are sorbitan sesquioleate, sorbitan dioleate, sorbitan trioleate, pentaerythritol dioleate and pentaerythritol trioleate. Other surfactants found operative in the composition disclosed, but deemed by the patentee to be somewhat less efficient, include alkenyldimethylethyl ammonium bromide; di"coco"dimethyl ammonium chloride quaternary imidazolinium salt (from stearic acid); glyceryl monooleate; glyceryl dioleate; glyceryl trioleate and polyglyceril ester of oleic acid. Nevertheless, a host of other surfactants were discovered to be inoperative in the invention due to the fact that they were either insoluble in the composition, or they did not improve the wettability of the composition as it tended to "bead" when applied to the surface of the nail.
In U.S. Pat. No. 4,349,605, there are disclosed radiation curable polymeric compositions having flame retardant properties which comprise copolymers of ethylene and a comonomer which may be vinyl ester or an acrylate or a methacrylate, a hydrated inorganic filler, an alkoxy silane, and a lubricant comprising lauric acid and ethylene bis-stearamide. The filler can, if desired, be silane treated, rather than adding the filler and silane separately to the composition. Such polymeric compositions are disclosed to be preferably cured by radiation means, although cross-linking of the polymers can also be achieved by chemical crosslinking or thermal crosslinking. According to the patentees, such polymeric compositions will hold very large amounts of filler and still provide high flexibility and a high degree of crosslinking. This is deemed by the patentees to be quite surprising as high flexibility and high crosslinking are generally incompatible, as are high crosslinking and high filler loading (which implies low crosslinkable polymer content). In compositions used for coating, e.g., extruding onto electrical wire and cables, best results, according to the patentees, are obtained when from 44 to 80% by weight of filler in the composition, or 22 to 59% volume of filler, preferably 50 to 57% by weight of filler in the composition, or 26 to 32% volume of filler are employed.
U.S. Pat. No. 4,390,401 discloses ultraviolet curable coating compositions which comprise polyunsaturated polyacrylates or methacrylates and as a wetting agent and adhesive promoter an acrylate or methacrylate of a polyalkylene oxide derivative of a mono hydric alkyl/aryl phenol.
Others, in addition to the patentees in U.S. Pat. No. 4,349,605, have disclosed using a pretreated filler in a polymeric composition. Thus, in U.S. Pat. No. 2,952,595, it has been disclosed that vinyltriethoxysilane treated filler, e.g. hydrated silica, in amounts from about 11 to 34% by weight of the composition or 10 to 30% volume of filler, enhances the beneficial effect that radiation treatment has on the filled polyethylene. Thus, with such treated fillers the impact strength of the compositions has been improved, and the brittle point was lowered, in addition to improved torsional hepteresis.
In Japanese Patent No. 56147846 (Matsushita Electric Works) there is disclosed a photocurable polyester resin composition for the production of thick sheets which comprises unsaturated polyester resin, a photopolymerization initiator, and an inorganic filler, e.g. calcium carbonate, calcium silicate, titanium oxide, aluminum oxide, talc, clay, alumina, calcium hydroxide, and magnesium carbonate, coated with a surfactant. Those surfactants specifically disclosed are stearic acid, lauric acid, rosin, lignin, and cationic surfactant. According to the abstract, the treated filler is used at 16.6 to 80% by weight of the composition. The surface-treated filler is claimed to permit UV to penetrate the composition to deep inside, allowing production of thick sheets. Other patents have cited the use of surfactants in combination with filler or surfactants in combination with polymer. Of interest are U.S. Pat. Nos. 4,510,175; 4,603,158; 4,349,605; 4,537,940; 4,407,984; and 4,459,193.
In U.S. Pat. No. 4,510,175, the patentee discloses preparing a slip composition consisting of ceramic powder, polymer, solvent, and surface active agent. The highest volume % of ceramic powder is 40%. The ceramic slip is prepared by ball milling the polymer, solvent, surfactant and ceramic powder. Slurry is then casted in layered sheets; with each successive sheet being dried before application of the next sheet. The final green ceramic composite is heated to 500 degrees C.; whereby the polymer is decomposed, and the ceramic body densified at 1300-1400 degrees C.
U.S. Pat. No. 4,603,158 describes an optically clear elastomer composition that was formed by molding the elastomer into desired shapes. The constituents of the molded article are an elastomer, fumed silica, coupling agent, antioxidant, thermal curing agents, and a phosphate salt used as an antistatic agent. Fumed silica has been added as a filler. Its volume % is 10-25.
Two patents, U.S. Pat. Nos. 4,349,605 and 4,407,984, use as a specific composition an acrylate, silane treated filler, two or more surfactants, and photoinitiator. The former patent is a solid composition while the latter is a thick paste that requires the addition of solvent to aid in premixing the constituents together. Both patents disclose the use of surface active agents. The surface active agents are acidic and basic. Both acidic and basic agents are added in the composition to disperse identically charged fillers In many examples, more than two dispersants are needed. Of particular importance is the use of a high volume % filler in U.S. Pat. No. 4,407,984. However, the filler is added to the composition with the aid of a solvent, methylene chloride.
Coincidental to the above patent by Ratcliffe is another patent by him, U.S. Pat. No. 4,459,193. In both patents the constituents are the same with one exception. In U.S. Pat. No. 4,459,193, the inventor does not use a surface active agent. The method of addition is the same. Likewise, the solvent, methylene chloride, is used as the medium for incorporating the solid. The solvent eventually evaporates and the composition forms a thick paste, as later disclosed in his patent; utilizing both acidic and basic surface active agents to simultaneously disperse identically charged fillers, i.e., fillers with the same pH.
UV curable phosphate acid ester monomers were described in detail in U.S. Pat. No. 4,537,940. The acid esters were combined with methacrylates, methacrylate powder, silane treated quartz, and a thermal initiator. Formulated systems were used as two part thermal dental adhesives.
None of the prior art of which we are aware provides a composition such as we have invented and disclosed and claim herein.